1. Field of the Invention
The present invention relates to an image forming apparatus based on an electrophotographic system, specifically to a cleaner-less image forming apparatus without using cleaning means for cleaning residual toner.
2. Description of the Related Art
In an image forming apparatus based on the electrophotographic system, recording apparatuses wherein a development unit collects residual toner therein while developing an image (named hereinafter the cleaner-less recording apparatus) have been known, for example, in Japanese patent laid-open No. SHO 59-133573 publication document and SHO 59-157661 publication document. In these documents, a basic concept of the cleaner-less image forming apparatus is disclosed. The outline of the concept is summarized as follows. In electrophotographic printers such as laser printers, the reversal development method has been widely used.
In the reversal development method, toner particles which are charged with the same polarity as an electrostatic latent image holding member (photosensitive material) are used, the toner particles being adhered to the portions where electric charge is absent or where the amount of electric charge is low on the electrostatic latent image holding member; the toner particles being not adhered to the portions where electric charge is present thereon. To selectively adhere toner particles, voltage V.sub.b (.vertline.V.sub.l .vertline.&lt;.vertline.V.sub.b .vertline.&lt;.vertline.Vo.vertline.) which is between voltage Vo of the charged portion on the electrostatic latent image holding member and voltage V.sub.l of the non-charged portion thereon is applied to the toner holding member of the development unit. The electric field of the charged portion prevents the toner from being adhered to the electrostatic latent image holding member. On the other hand, the electric field of the non-charged portion causes the toner to be adhered to the electrostatic latent image holding member.
The toner adhered on the electrostatic latent image holding member is transferred to an image holding material by a known transfer unit. In the image transfer process, all the toner particles are not transferred to the image holding material. Instead, the residual toner distributively stays in an image shape on the electrostatic latent image holding member.
In the conventional electrophotographic apparatus, the residual toner is collected by a cleaner and the electric charge which stays on the electrostatic latent image holding member is cleared by a charge clearing lamp. After that, a latent image forming process (consisting of a charge equalization process by a charger and an exposure process by a light beam) is performed. On the other hand, in the cleaner-less image forming apparatus, the residual toner is collected in the development unit while performing a development process without using such a cleaner. Strictly speaking, since the residual toner which is present in the charged portion (non-exposure portion or non-image portion) of the latent image formed in the light beam exposure process is securely charged with the same polarity as the latent image by the charger, an electric field which prevents the toner particles from moving from the toner holding member to the electrostatic latent image holding member, namely, the electric field produced by a potential of Vo and V.sub.b, the residual toner is moved to the toner holding member side. At the same time, the residual toner which is present in the non-charged portion (exposure portion or image portion) is urged by a force which works from the toner holding member to the electrostatic latent image holding member. Thus, the residual toner stays on the electrostatic latent image holding member, new toner particles being moved from the toner holding member to the non-charged portion. Consequently, while an image is developed, the residual toner is cleaned.
As described above, in the cleaner-less image forming apparatus, since it is not necessary to provide the cleaner and a waste toner box which stores collected toner (waste toner), the apparatus can be easily and simply structured in a small size. In addition, the residual toner is collected in the development unit and then reused therein. Thus, no waste toner occurs and thereby the cost performance increases. In addition, since the surface of the electrostatic latent image holding ember is not slid by a cleaning blade, the life of the electrostatic latent image holding member can be prolonged. Thus, the cleaner-less image forming apparatus has many benefits like above. However, in the conventional cleaner-less image forming apparatuses, ghost images may occur due to the following causes.
Firstly, in a high humidity environment, a paper as the image holding material absorbs moisture and thereby the resistance decreases. Thus, generally there is a tendency for the transfer efficiency to decrease and for a significant amount of toner to stay on the electrostatic latent image holding member. In other words, when the resistance of the transfer recording paper decreases due to absorption of moisture or the transfer conditions are not optimum values due to disconformity of the material and thickness, there is a tendency for the toner to stay massively on the electrostatic latent image holding member. In the worst case, in the transfer process, the transfer recording paper is not contacted with the electrostatic latent image holding member due to a wrinkle of the recording paper and thereby the desired transfer operation may not take place.
When the amount of residual toner becomes excessive, the required cleaning operation cannot be conducted in the development position and the residual toner stays in the non-image section. Thus, a positive ghost appears in the white portion of the transfer image which is named the positive ghost or positive memory hereinafter.
Secondly, when the amount of residual toner becomes excessive, since the residual toner shields the light beam in the exposure process, an insufficient attenuation of the potential of the electrostatic latent image holding member takes place. In this portion, since the development voltage becomes V.sub.b -Vl'(Vo&lt;Vl'&lt;V.sub.l) which is smaller than the development voltage V.sub.b -Vl of the surrounding exposure portion. Thus, the amount of toner which is transferred from the toner holding member to the latent image holding material becomes smaller than other surrounding portions. Thus, at the image portion of the transfer image, a white drop image takes place by the residual toner image. The white drop image is named the negative ghost or negative memory hereinafter. This phenomenon remarkably occurs in a half tone image consisting of a combination of blind spot images and line images.
To solve the above problem, for example, Japanese patent laid-open No. SHO 62-203183, No. SHO 64-50089, and SHO 64-50089 publication documents disclose that by applying a voltage to an electro-conductive brush, that slightly contacts an electrostatic latent image holding member, the ghost can be cleared. In other words, by applying a voltage whose polarity is reverse of that of the toner being charged to the electro-conductive brush by a DC power supply, the residual toner is attracted to the electro-conductive brush by means of the Coulomb's force. On the other hand, at the non-image portion of the electrostatic latent image holding member, the toner which is positively charged is emitted from the single electrode type brush. In such a manner, the residual toner is equally distributed. Thus, the amount of the residual toner on the electrostatic latent image holding member is remarkably reduced, thereby preventing the ghost from occurring.
However, in the method where the residual toner is attached and removed by the above electro-conductive brush, the following problem will arise.
Firstly, although for a laser printer, a high resolution image is required, because of excessive residual toner which shields a light beam, the memory phenomenon often occurs. Thus, the allowable toner density should be very low. However, it is very difficult to satisfy such a requirement which reduces the amount of the residual toner to the allowable level.
Secondly, for an image forming apparatus, in various environmental conditions, images are transferred to various recording papers. In this situation, the charging polarity and the charging amount depend on the resistance of each recording paper. For example, when the resistance of the recording paper is low, the positive electric charge which is applied from a transfer unit 5 to the recording paper is moved to the direction of the thickness of the recording paper and arrives at the toner particles on the electrostatic latent image holding member. Thus, the polarity of the toner is reversed and thereby it is positively charged. In addition, the surface of the electrostatic latent image holding member is positively charged. Thus, the single electrode type brush produces a repulsive force against the residual toner rather than attracting it. Thus, the function for preventing the memory phenomenon from occurring cannot be performed.
Thirdly, the residual toner is emitted to the non-image portion. Particularly, when the single electrode type brush excessively attracts the residual toner, the amount of emitted toner increases, thereby shielding the exposure beam. Thus, an image defect may occur.
Fourthly, since the single electrode type brush has a limit for attracting and holding the toner, when the amount comes to the limit, the brush does not attract the residual toner. Thus, the memory protection function does not work.
This point will be described in detail. FIG. 1 shows the amount of residual toner adhered (curve A) and the amount of charging against transfer corona voltage V.sub.t (curve B). In other words, when the transfer corona voltage V.sub.t becomes approximately 5.0 kV, the transfer efficiency becomes maximum and thereby the amount of the residual toner adhered becomes minimum. When the transfer corona voltage V.sub.t becomes approximately 5.0 kV, the charging polarity of the residual toner is reversed from the negative to the positive and the amount of charging of the residual toner becomes approximately 0. In other words, the electric charge which is transferred from the transfer unit to the recording paper is moved in the direction of the thickness of the recording paper and arrives at the toner on the electrostatic latent image holding member. Thus, the toner which has been negatively charged is gradually neutralized by the positive electric charge. Consequently, it can be considered that the transfer corona voltage V.sub.t is a factor for restricting the electric charge which is emitted to the toner. For example, by keeping the transfer corona voltage V.sub.t constant and changing the material, thickness, moisture rate, and so forth of the paper, even if the resistance is changed, the same result takes place.
For example, FIG. 2 shows the relationship between the residual toner density after the development unit cleans the surface of the electrostatic latent image holding member while developing an image, the unit applying a predetermined corona voltage to the transfer unit to transfer the image to the transfer paper (curve C) and after the toner is passed though the single electrode type brush (curve D). The residual toner density after the toner image is transferred to a photosensitive drum 1 was measured by using a method disclosed in Japanese patent laid-open No. SHO 64-50089 publication document. For example, the toner image on the photosensitive drum 1 is transferred to a mending tape. The mending tape is adhered on a white paper and the reflection density is measured (when the toner is absent, the reflection density of the mending tape is approximately 0.11, which is named the tape density).
As shown in FIG. 2, the residual toner density becomes minimum when the corona voltage V.sub.t is 4.9 kV (approximately 0.23). In the vicinity of the voltage, the density increases. On the other hand, after the toner is passed through the single electrode type brush, the residual toner density becomes minimum when the corona voltage V.sub.t is 4.4 kV. When the corona voltage V.sub.t is 4.9 kV or more, the residual toner density after the toner is passed through the single electrode type brush follows with the curve of the residual toner density after the toner is transferred. This is because in the vicinity of V.sub.t=4.9 kV (the amount of residual toner adhered after the toner is transferred becomes minimum), the charging amount of the residual toner after the toner is transferred is nearly zero and in the vicinity the electric charging polarity is reversed as shown in FIG. 1. FIG. 3 shows a descriptive diagram showing this phenomenon. The figure shows the attraction and emission of the toner and the surface voltage Vo of the electrostatic latent image holding member at the contact portion between the single electrode type brush and the electrostatic latent image holding member and Vo of the surface voltages of the image portion and the non-image portion of the electrostatic latent image holding member.
In the case that the transfer corona voltage V.sub.t is 4.4 kV:
The density decreases near to density 0.11 of tape on a white paper (named the tape density hereinafter) where the single electrode type brush attracts the residual toner and the toner is not adhered on the surface of the electrostatic latent image holding member. When, in the single electrode type brush position, the surface potential of the electrostatic latent image holding member slightly changes in the positive direction both for the image portion and the non-image portion. Since the residual toner is negatively charged as shown in FIG. 1, the residual toner is attracted by the single electrode type brush where a positive voltage is applied. At the non-image portion, the positively charged toner particles (part of toner particles are positively charged by frictions between toner particles, between toner particles and the single electrode type brush, and between the toner particles and the electrostatic latent image holding member, by charge injection, and by discharging) are emitted onto the electrostatic latent image holding member and the electric charge is moved among the single electrode type brush, the electrostatic latent image holding member, and the toner particles. Thus, after the toner is passed through the single electrode type brush, the residual toner is equalized and the surface voltage of the electrostatic latent image holding member nearly becomes constant.
In the case that the transfer corona voltage is 4.9 kV:
Since the single electrode type brush almost does not attract the toner, there is nearly no difference between the residual toner density after the toner is transferred and that after it is passed through the single electrode type brush. At the time, in the single electrode type brush position, the surface potential of the electrostatic latent image holding member changes in the positive direction both for the image portion and non-image portion since positive electric charge is fed from the transfer unit (particularly, for the non-image portion, the potential remarkably changes and thereby the potential between the image portion and the non-image portion is reduced). As shown in FIG. 1, since the charging amount of the residual toner is nearly 0 and the potential between the single electrode type brush voltage V.sub.w and the voltage of the image portion is small, the Coulomb's force which acts on the residual toner is small and thereby the residual toner is still adhered on the surface of the electrostatic latent image holding member. On the other hand, at the non-image portion, since the toner is not attracted by the single electrode type brush, the amount of toner positively charged is small. In addition, since the potential between the single electrode type brush and the non-image portion is small, the amount of toner which is emitted to the single electrode type brush is small. Moreover, since the amount of electric charge which moves is small because of the above reason and the surface voltage slightly changes, even after the toner is passed through the single electrode type brush, the residual toner is not equalized.
In the case that the transfer corona voltage V.sub.t =5.4 kV:
As shown in FIG. 2, there is almost no difference between the residual toner density after the toner is transferred and that after it is passed through the single electrode type brush. In other words, like the case of the transfer corona voltage V.sub.t =4.9 kV described above, the surface voltage of the electrostatic latent image holding member remarkably changes in the positive direction. On the other hand, as shown in FIG. 1, since the charging polarity of the residual toner is positive, the Coulobm's force acts so that the toner is adhered on the surface of the electrostatic latent image holding member. Thus, even after the toner is passed through the single electrode type brush, the residual toner is not equalized.
As described above, in the memory phenomenon protection or memory clearing method using the single electrode type brush (positive voltage is applied), only in the range where the charging polarity of the toner is negative, the toner is attracted and emitted and thereby in the vicinity where the charging amount becomes 0 (where the residual toner density after the toner is transferred becomes a minimum value), the residual toner density does not change. In other words, in the conventional method, for the residual toner density which is required for forming images in high resolution, equalization of the residual toner, memory phenomenon protection, and memory clearance cannot be satisfactorily accomplished.